3GPP Long Term Evolution (LTE) is a standard for mobile phone network technology. LTE is a set of enhancements to the Universal Mobile Telecommunications System (UMTS), and is a technology for realizing high-speed packet-based communication that can reach high data rates on both downlink and uplink channels. In LTE, transmissions are sent from base stations, such as Node Bs (NBs) and evolved Node Bs (eNBs), to mobile stations (e.g., user equipment (UE)). These transmissions are sent using orthogonal frequency division multiplexing (OFDM), which splits the signal into multiple parallel sub-carriers in frequency.
As illustrated in FIG. 1, the basic unit of a transmission in LTE is a resource block (RB) 100, which in its most common configuration consists of 12 sub-carriers 104 and 7 OFDM symbols 108 (i.e., one slot). An OFDM symbol 108 may include a cyclic prefix 106. A unit of one sub-carrier and one OFDM symbol is referred to as a resource element (RE) 102. Thus, an RB may consist of, for example, 84 REs in a 12×7 configuration.
An LTE radio sub-frame may be composed of multiple resource blocks in frequency and two slots in time, with the number of RBs determining the bandwidth of the system. Two RBs in a sub-frame, that are adjacent in time, for instance as shown in FIG. 3, may be referred to as an RB pair 300. In the time domain, an LTE downlink transmission may be organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized sub-frames of length Tsub-frame=1 ms.
LTE may be deployed in a number of configurations such as Multiple-Input, Multiple-Output (MIMO) radio systems. An exemplary MIMO system including a base station 502, such as an eNB, and user equipment 504 is shown in FIG. 5. When a signal is transmitted by the eNB 502 in a downlink, i.e., the link carrying transmissions from the eNB to the UE 504, a sub-frame may be transmitted from multiple antennas 506,508 and the signal may be received at a UE 504, which has one or more antennas. The radio channel distorts the transmitted signals from the multiple antenna ports.
Due to the multiple paths and conditions on each channel, in order to demodulate a transmission on the downlink, the UE 504 relies on reference symbols (RS) that are also transmitted on the downlink. An RS may be understood as one or more REs carrying pre-defined symbols. These reference symbols and their position in the time-frequency grid are known, or otherwise determined, by the UE. Thus, the RSs can be used to determine channel estimates by measuring the effect of a specific radio channel on these symbols.
According to the LTE standard, transmissions from an eNB are sent from “antenna ports” rather than antennas. An antenna port may be understood as a virtual antenna, which can further be associated with a reference symbol RS. Thus, when a UE measures the channel from an antenna port to the receiver antenna, which physical antenna elements were used for the transmission is irrelevant for the UE. The transmission on an antenna port may originate from a single physical antenna element or may be the combination of signals from multiple antenna elements.
In certain instances, the use of transmit pre-coding can be used to direct transmitted energy towards a specific receiving UE. This may be accomplished by using all available antenna elements to transmit the same message, with different phase and/or amplitude weights applied at each antenna element. Since the reference symbol associated with each antenna port also undergoes the same pre-coding operation with identical pre-coding weights as the data, the transmission uses a single virtual antenna/single antenna port, and the UE need only perform channel estimation using a single RS.
There are several broad types of RSs used in LTE. A first type of RS is one that can be used by all UEs, and thus, have wide cell area coverage. One example of this type of reference symbol is the common reference symbol (CRS) that is used by UEs for various purposes, including channel estimation. Presently, these CRSs are defined so that they occupy certain pre-defined REs within the transmission sub-frame, regardless of whether there is any data being sent to users or not. For example, as shown in FIG. 2, a sub-frame 200 may include a control region, control signaling, and reference symbols 202. Reference symbols 202 may be a CRS used by a UE in the communication network.
A second type of RS is a UE-specific reference symbol, which is intended specifically for use by only a certain UE or set of UEs. Presently, these UE-specific RSs are transmitted only when data is transmitted to a certain UE. When pre-coded for a specific UE or set of UEs, the RS does not reach all parts of the cell, but only those parts of the cell where the UEs of interest (to which the data in intended) are located.
In LTE, UE-specific reference symbols are included as part of the resource blocks that are allocated to a UE for reception of user data. The exemplary use of UE-specific RSs in LTE is shown in the RB pair of FIG. 3, which includes UE-specific RSs R7 and R9.
Further, messages transmitted over a radio link to UEs in an LTE network can be broadly classified as control messages or data messages. Control messages are used to facilitate the proper operation of the system as well as proper operation of each UE within the system. Control messages could include, for example, commands to control functions such as transmitted power or additional signaling with RBs. Examples of control messages include, but are not limited to, the physical control format indicator channel (PCFICH) which carries configuration information of the control region size; the physical downlink control channel (PDCCH) which, for example, carries scheduling information and power control messages; the physical HARQ indicator channel (PHICH), which carries ACK/NACK in response to a previous uplink transmission; and the physical broadcast channel (PBCH), which carries system information.
In LTE Rel-10, control messages are demodulated using the CRS. The first one to four OFDM symbols, depending on the configuration, in a sub-frame are reserved for control information, for instance as shown in FIG. 2. Control messages of PDCCH type are transmitted in multiples of units called control channel elements (CCEs), where each CCE contains 36 REs.
Presently, data messages may be transmitted to users in RBs, which carry UE-specific RSs. These RSs may be used by the UEs to demodulate the data messages. The use of UE-specific RSs allows a multi-antenna eNB to optimize the transmission using pre-coding of signals being transmitted from the multiple antennas so that the received signal becomes stronger at the UE and consequently, the data rate of the transmission can be increased.
Similarly, Rel-10 of LTE also defines a control channel called the R-PDCCH for transmitting control information to relay nodes. The relay node receiving the R-PDCCH can use relay node (RN) specific reference signals to improve link performance. Adoption of the same principle of transmission as for the R-PDCCH has been considered by allowing the transmission of generic control messages to a UE using such transmissions based on UE-specific RSs.
Control messages could be categorized into those types of messages that need to be sent only to one UE (UE-specific control) and those that need to be sent to all UEs or some subset of UEs (common control) within the cell covered by the eNB. In the R-PDCCH, RN-specific messages are demodulated using RN-specific RS, whereas common control messages are demodulated using the CRS. The use of CRS has certain disadvantages. First, since CRS density is high within each RB (see FIG. 2), and each antenna requires its own CRS, orthogonal to the CRS of the other antenna ports, the overhead generated by the CRS can be quite high depending on the number of antennas used for transmission (roughly 9.5% for 2 antenna port transmission). Second, the transmission of CRS does not scale with the amount of user data being transmitted in the system. Thus, the mandatory transmission of CRS leads to an energy inefficient system, especially since they must be always on, even if there is no data transmission. Recent analyses have shown that a vast majority of sub-frames transmitted in an LTE system have no data or control messages transmitted in them.
Since common control signals are intended to be reached by all UEs in the cell, a wide coverage of the transmit radiation pattern must be used. Therefore, they are transmitted using either a single port transmission or using transmit diversity. Existing systems send messages that are common to multiple UEs by a wide cell coverage transmission format that requires channel estimation using the CRS. Transmission of reference signals such as the CRS that do not scale with the volume of control messages being sent is energy inefficient and also has impacts on performance due to the additional overhead accrued. Existing systems also do not provide a single unifying transmission scheme for both common control and UE-specific control messages.
A further problem exists regarding how to transmit common control signals with wide area coverage while utilizing multiple antenna ports for antenna diversity to enhance the robustness of the control channel, which is of paramount importance for stable system operation.
Another problem with existing systems is a lack of flexibility in the eNB to either transmit UE-specific control messages using UE-specific reference symbols so that these transmissions can be pre-coded to optimize the transmission for the UE, while at the same time using the same type of RS to transmit common control messages to a larger group of UEs or UE-specific control messages to a UE. Therefore, a problem exists regarding how to allow a eNB to seamlessly transition between sending control messages that are common to multiple UEs and that are specific to a UE, while minimizing the changes to eNB and UE operations.
Accordingly, there is a need for a method and device for improving transmission techniques from a base station with multiple antenna ports to a UE, using UE-specific reference symbols.